The two became "poster girls" for the militant movement with pictures posted on social media showing them in full-length burkas with armed men.

A note left behind by the girls for their families said: "Don’t look for us. We will serve Allah — and we will die for him."

They are believed to have become radicalised by reading jihadi literature on the internet and attending a local mosque in Vienna.

Selimovic and Kesinovic posted this photo to social media

Peter Slanar, the headmaster of the school the teenagers attended, told The Express: "I would have said a year ago that they were perfectly normal teenage girls, but then the older, Samra, started sending pictures of herself in the Muslim headscarf to fellow pupils and was trying to convert everybody to her point of view.

"There were vandalism incidents in which tables and walls had the words 'I love Al Qaeda' written on them.

"We had to act when they started saying that America was to blame for the September 11 attack. That was simply going too far."

The men told Professor Peter Neumann of King's College that they regret their decision after becoming frustrated that rival rebel groups are fighting among themselves, instead of targeting President Assad’s forces.

Profiles on Interpol of the two teenagers who went missing in April

Professor Neumann told the Times newspaper they have been in contact with a number of British jihadis who want to come back to the UK but feel they cannot do so for fear they will be jailed.

“We came to fight the regime and instead we are involved in gang warfare. It’s not what we came for but if we go back [to Britain] we will go to jail,” one jihadist, claiming to represent 30 others, told researchers.

He said the Government should set up a "de-radicalisation programme" for those willing former jihadis, echoing calls by Labour leader Ed Miliband who last month suggested a mandatory programme of de-radicalisation for those involved on the fringes with Islamic State (IS).